1. Field of Invention
The present invention relates generally to a method of forming a pressure tight sensor housing assembly so as to eliminate pressure xe2x80x9cleakagexe2x80x9d.
2. Description of the Related Art
As set forth in the patent application cited above, in principle, a pressure sensor consists of a spring element (measuring element) and a measurement (sensor) device. Combined together as a unit, the measuring element and the measurement device can be used to provide pressure measurements. Commercially available pressure sensors can employ membranes or diaphragms as spring elements, along with piezo-resistive resistors to provide a signal representing the deflection of the membrane or diaphragm. Spring elements as membranes are unfavorable at high pressures because they are sensitive to effects related to the clamping in a substrate with a transition to materials having an unequal modulus of elasticity. The stress detected in such membranes will be a combination of compressive and tensile stresses. If the tensile stresses becomes sufficiently high, a break can occur. Typically, the signals of the stresses in membranes and/or sensors are linear functions of pressure. However, large deformations in membranes/sensors will cause non-linear stress to pressure signals.
One of the advantages in the sensor of U.S. patent application Ser. No. 09/125,775 (shown in FIGS. 1-3 of the present drawings) is that the geometry of the measuring element serves to overcome the aforementioned shortcomings. Additionally, by using micromechanical manufacturing techniques, it is possible to produce measuring elements according to the invention with very small dimensions. Producing very small dimension measuring elements are particularly advantages for high-pressure measurements. Also, the geometry of the measuring element or sensor is advantageous because the axial length of the sensor is an order of a magnitude times its lateral length, thereby making mounting, integrating and/or packaging of the sensor more practical.
In a typical embodiment, a sensor is enclosed in a sensor housing assembly for use in high pressure applications, including measuring pressures in a diesel fuel injection system. Typically at very high pressures of the order of 30,000 psi (approximately 2200 atmospheric pressures) and at high temperatures, the housing assembly should aid the sensor in providing accurate pressure measurements. Thus, these accurate pressure measurements that can be provided by the sensor, are achieved if the housing assembly is sufficiently pressure tight. That is, there should be no xe2x80x9cleakagexe2x80x9d in pressure due to porosities in the sensor assembly.
Accordingly, it is an object of this invention to provide a method for incorporating a sensor within a sensor housing assembly.
It is another object of this invention to provide a method for incorporating a sensor within a housing assembly to withstand very high pressures and high temperatures.
It is yet another object of this invention to provide a method for incorporating a sensor within a pressure tight housing assembly.
These and other objects and advantages are attained by placing the elongated sensor in a pressure tight enclosure. In accordance with one illustrative embodiment of the invention, a KOVAR alloy fitting forms a portion of the assembly.
Typically, KOVAR alloy is used for metallic parts which make hermetic seals with glass based and ceramic based materials in view of its thermal expansion properties which are compatible with those of glass or ceramic based materials. The interface between the KOVAR alloy and the sensor may be a glass sleeve in one embodiment. In a preferred embodiment a glass paste is used. This combined KOVAR-glass-sensor system is heated to a temperature of approximately four hundred degrees Celsius. Subsequently, a final sealer is applied to this system. In the present embodiment, the sealer is an epoxy resin which is applied to this system by means of a high pressure gas, the gas being pressurized at about five thousand psi (approximately 360 atmospheric pressures) of air pressure for about thirty minutes. During the curing phase the combined KOVAR-glass-sensor-resin system is cured at approximately 150 degrees Celsius for about one hour. After this method was employed, the sensor system is able to accurately measure pressures of about thirty thousand psi (approximately two thousand and two hundred atmospheric pressures) at various temperatures for a long duration. Additionally, no further xe2x80x9cmicro-leaksxe2x80x9d in pressure were observed with this invention.
The total time taken by the process from start to finish is less than about two hours.
From a broader standpoint the invention involves sealing a high strength, elongated hollow pressure sensing element into a high strength metal fitting. The metal fitting has a relatively higher thermal coefficient of expansion than the sensor element. An intermediate transitioning member having an opening which may be centrally located is fitted between the metal fitting and the sensing element. The thermal coefficient of expansion of the transition member lies between the thermal coefficients of the sensing element and the metal. The pressure sensing element extends through the opening in the transitioning member. Additionally, a filler or filling element is positioned in the space between the transitioning member and the sensing element. The filler has a thermal coefficient of expansion that lies between the thermal coefficients of the sensing element and the intermediate transitioning member. The combined system or assembly is then heated to thermally bond and seal the sensing element in place within the assembly. Subsequently additional sealing material such as an epoxy may be applied under pressure, and then cured to seal any possible residual openings in the assembly. There are at least two means to avoid failure caused by temperature cycling. The first means is by the gradual transitioning of the thermal coefficients of expansion. The second means is by increased contraction of the outer parts of the fitting versus the inner parts of the fitting upon cooling, thereby producing a squeezing and sealing effect.
The above described and many other features and attendant advantages of the present invention will become apparent from a consideration of the following detailed description when considered in conjunction with the accompanying drawings.